Amid Congressional calls for closer scrutiny of American aid to Pakistan in the wake of the arrest of five CIA informers, the United States has acknowledged "challenges" in relations with Islamabad.

Amid Congressional calls for closer scrutiny of American aid to Pakistan in the wake of the arrest of five CIA informers, the United States has acknowledged "challenges" in relations with Islamabad.

But security cooperation with Pakistan remains in both countries' best interests, US officials said on Wednesday when asked about a report that the Pakistani spy agency has arrested informers who helped US before the raid on Osama bin Laden's hideout on May 2.

Stressing the continued value of engagement with, and aid to, Pakistan, officials said the Obama administration is committed to working through what they termed challenges in the relationship.

At the White House, spokesman Jay Carney said US relations with Pakistan are "complicated", but that anti-terrorism cooperation with Pakistan is vital to American interests.

State department spokesman Mark Toner, meanwhile, said the parade of high-level US visitors to Pakistan since the May 2 raid underlines the commitment of the two countries to "work through" their problems.

"I think we've been upfront about challenges in the relationship. But we've also been consistent in saying that Pakistan and the US need each other," he said.

"We need to work through these challenges because it's in both of our long term, and short term frankly, interests to do so."

Even as US lawmakers expressed outrage over the arrest of five Pakistan CIA informers by Pakistan's spy agency, the top US military official defended ties with a key ally as "complicated" but vital.

"Some of the criticism is more than warranted" when it comes to the relationship with Pakistan, chairman of the Joint Chiefs of Staff, Admiral Michael Mullen said on Wednesday at a US Senate hearing.

Although the relationship with Pakistan is complicated, not dealing with the Pakistanis would likely mean the US would be out of the picture in Pakistan -- where the Afghan Taliban is believed to have regrouped -- for another five to 10 years, he said.

The US is in the midst of building a relationship with Pakistan, which was "badly broken" in the '80s and '90s. And we are back," Mullen said.

"Nobody's worked that harder than me, very frankly, with the leadership -- and it's a conscious decision, I think, that we have to make," he said. "If we walk away from it, it's my view it'll be a much more dangerous place a decade from now, and we'll be back."

"I think, a goal that we must continue to pursue -- or the danger associated with a country that’s got a nuclear arsenal, that lives next to a country that they view as an existential threat, it’s just a matter of time before we’re back," Mullen said.